1. Field of the Invention
The present invention relates to a surface acoustic wave device for use as, for example, a band pass filter and, more particularly, to a surface acoustic wave device having a first surface acoustic wave resonator filter connected in parallel to a second surface acoustic wave resonator filter.
2. Description of the Related Art
Various types of surface acoustic wave devices including a plurality of surface acoustic wave resonator filters that are connected in parallel to one another have been proposed. For example, Japanese Unexamined Patent Application Publication No. 6-334476 (hereinafter referred to as Patent Document 1) discloses a surface acoustic wave device including a plurality of IDT electrodes that are provided on a quartz substrate and a first surface acoustic wave resonator filter including the IDT electrodes that is connected in parallel to a second surface acoustic wave resonator filter also including the IDT electrodes. Herein, filter characteristics of the parallel-connected surface acoustic wave resonator filters are shifted towards each other to achieve a wide-band filter characteristic of a surface acoustic wave device.
FIG. 10 shows an example of a filter characteristic of such a known surface acoustic wave device.
On the other hand, Japanese Unexamined Patent Application Publication No. 7-58585 (hereinafter referred to as Patent Document 2) discloses a surface acoustic wave device that is similar to the surface acoustic wave device disclosed in Patent Document 1. However, the relationship of filter characteristics of parallel-connected surface acoustic wave resonator filters disclosed in Patent Document 2 differs from that in Patent Document 1. That is, with reference to FIG. 11, a first characteristic and a second characteristic of the surface acoustic wave resonator filters are shown by dashed lines A and B, respectively. The overall filter characteristic of the surface acoustic wave device is shown by a solid line. As can be seen from the dashed lines A and B, each surface acoustic wave resonator has three different resonance points of a longitudinal mode. In the surface acoustic wave device in Patent Document 2, two resonance points on the high frequency side of one surface acoustic wave resonator filter are brought into coincidence with two resonance points on the low frequency side of the other surface acoustic wave resonator filter. Thus, the overall filter characteristic of the surface acoustic wave device is obtained.
To construct a longitudinally coupled surface acoustic wave resonator filter, a plurality of surface acoustic wave resonator filters are connected in parallel on a piezoelectric substrate, as disclosed in Patent Documents 1 and 2, as is well known. Such a structure decreases an impedance of the surface acoustic wave filter. In addition, a wide-band filter characteristic is achieved by shifting the filter characteristics of the surface acoustic wave resonator filters towards each other, as disclosed in Patent Documents 1 and 2.
Unfortunately, although a wide-band filter characteristic is achieved by shifting the filter characteristics of the first and second surface acoustic wave resonator filters towards each other, an undesired ripple is produced in the overall filter characteristic.
On the other hand, as disclosed in Patent Document 2, overlapping of a plurality of resonance points in the filter characteristics of the first and second surface acoustic wave resonator filters suppresses the undesired ripple. However, even in this design, a ripple occurs sometimes due to variations in manufacturing under some design conditions. That is, the ripple may occur if the overlapped resonance points are offset and a continuity of the phase is lost. In particular, in the case of a high Q factor of the resonance point, namely, in the case of a steep change in the phase, a slight offset of the overlapped resonance points significantly shifts the phase, thereby generating a ripple.